The present invention relates to methods and apparatus for generating a reference television signal derived from a continuous wave (CW) signal used in a local TV station, especially, for automatically adjusting the phase with respect to a reference television signal from a key TV station.
In national television broadcasting, key TV stations usually located in major metropolitan areas distribute TV programs to local TV stations of a national network. In this case, the vertical sync signals of the TV signals from the key TV station and local TV stations must be synchronized to avoid disturbing the video picture when it is switched from a key TV station's video to a local TV station's video and vice versa. Therefore, the local TV stations have their black burst (BB) signal (hereinafter local TV station BB signal) synchronized with a BB signal from the key TV station (hereinafter key TV station BB signal)
For example, the local TV station may continuously receive the key TV station BB signal to obtain GenLock with the local TV station BB signal wherein the GenLock is disclosed in U.S. Pat. No. 4,751,565 for example. The non-stop receiving of the key TV station BB signal, however, is not economical because it uses a commercial communication line. Then, it sometimes receives the key TV station BB signal for the Genlock to synchronize with the local TV station BB signal to reduce the communication cost.
A conventional method of synchronization between the key and local TV station BB signals is described below with referring to FIG. 1. The local TV station has a high precision oscillator 10 that generates a continuous wave (CW, or a sine wave) having a frequency that is very accurately maintained. A reference television signal generator 12 generates a black burst signal (local TV station BB signal) of which period is known and maintained very accurately with respect to the precision CW signal received from the high precision oscillator 10. The reference television signal generator 12 has a delay line,which a user can adjust using buttons 13 to adjust the delay of the output BB signal to accomplish the phase adjustment. A waveform monitor 14 receives a key TV station BB signal at the first input channel and the local TV station BB signal from the reference television signal generator 12 at the second input channel. A screen 16 of the waveform monitor 14 shows waveforms of the first and second channels. The user manually operates the phase adjust buttons 13 while watching the screen 16 to align the phases of the vertical sync signals of the key and local TV station BB signals. It produces a local TV station BB signal with a vertical sync signal that is aligned with that of the key TV station BB signal. After the phase alignment, the local TV station may disconnect the commercial line to receive the key TV station BB signal. Since the CW signal is very accurate, the local TV station BB signal keeps the synchronization with the key TV station BB signal without the phase alignment process for a while, which is called “CW lock”. The phase alignment process with the key TV station BB signal may be repeated at predetermined intervals to keep the key and local TV station BB signalssynchronized.
Alternatively, a frame pulse may be produced instead of the BB signal from the reference television signal generator 12 and the process may be applicable to any formats including SDI as well as NTSC. If the reference television signal generator 12 provides a signal for which the phase relationship with the key TV station BB signal is determined and the period is known, a local TV station BB signal could be easily derived from the signal.
Since the CW signal does not have a vertical sync signal etc., the CW signal does not provide a phase reference to determine the phase of a reference television signal, such as BB signal or frame pulse etc., derived from the CW signal. Therefore, manual phase alignment with the key TV station BB signal while watching the screen is necessary as shown in FIG. 1, which is troublesome. It would be easy to synchronize the key and local TV station BB signals if the local station always receives the key TV station BB signal through a commercial line, but it would not be economical. Therefore, what is desired is to generate a reference signal for a local TV station, such as a BB signal, a frame pulse, etc., that is automatically synchronized with a key TV station BB signal.